1. Field of the Invention
The present invention relates to a piezoelectric element suitable for rotary piezoelectric actuator and the like, and to a method for manufacturing the same.
2. Description of the Related Art
Information appliances are designed toward further downsizing and precision in recent years, which tendency increases the demand for actuators that achieve slight movement thereof. Those actuators require a piezoelectric element which can control movements of very small distances. Their applications include those for optical focal correction and tilt control, and for head of ink-jet printer or magnetic disk unit. Particularly for the magnetic disk units, their capacity increases year after year, and the requirement to increase the memory capacity per disk has increased. Accordingly, narrowing the track width of disk has become important. As a result, the small piezoelectric elements for the head actuators that increase the head positioning accuracy in the tracking direction are required to assure high reliability.
A piezoelectric element used in a head actuator of magnetic disk unit or the like generally has multi-layer structure as shown in FIG. 20. The piezoelectric element having that type of multi-layer structure is manufactured generally by the steps of sintering a stacked pile of green sheets 102 sandwiching an electrode layer 101 therebetween to integrate them, cutting them using a dicing saw or the like, and then forming an electrode 103 on each end face thereof. Regarding the piezoelectric element used in a head actuator of magnetic disk unit or the like, the thickness of each green sheet 102 is limited to several tens of micrometer to assure the desired displacement and owing to the dimensional limitation.
Furthermore, above-described head actuators are designed to decrease the applied voltage. To decrease the voltage, large displacement has to be attained while decreasing the applied voltage. For example, Patent Documents 1 through 3 disclose technologies to achieve the objective using materials having high piezoelectric constant.
On the other hand, there is significantly increased the intensity of electric field being applied along with the decreasing in voltage. In particular, the head actuators of magnetic disk units need a large control distance, which increases the intensity of electric field responding to the increased control distance. In addition, the head actuators are requested to have high reliability in a high temperature and high humidity environment because of their severe operating environments.
If, however, the head actuator adopts a conventional piezoelectric element shown in FIG. 20, continued operation thereof in a high temperature and high humidity environment extremely decreases the insulation resistance, and may lead to dielectric breakdown in a short time.
A cause of the phenomenon is conventionally presumed to be the exposure of the electrode layer 101, which constitutes the active part of the piezoelectric element. Based on the speculation, studies are conducted on the structure not exposing the electrode layer 101 and on the structure forming a resin protective film 125 to cover the electrode layer 101 on both side faces as shown in FIG. 21. Those structures, however, restrict the deformation of piezoelectric element because the portion not exposing the electrode and the resin protective film 125 act as a wall, thereby decreasing the magnitude of displacement, or narrowing the movable range.
In addition, there is a possible measure of forming a moisture-proof film over the whole surface area of the piezoelectric element. However, it becomes difficult for the piezoelectric element to adhere to the suspension and the slider on fabricating the magnetic disk unit owing to the high water repellency of the moisture-proof film.
There is another measure as described in Patent Document 4. According to the description, the whole surface area of a Head Gimbal Assembly (HGA) structured by fixing a piezoelectric element to a suspension and a slider is covered by a protective film. Although the measure forms the protective film also on the floating surface of the slider, the existence of fine irregularities on the floating surface prevents the formation of protective film uniformly along the profile of the irregular surface, thus making it difficult to adequately control the floating amount of slider above the magnetic disk. In addition, since the protective film is formed also on the magnetic head, the read-write characteristic of the magnetic signal fluctuates.
As described above, the piezoelectric elements of the related arts are difficult to assure high reliability on applying them to the magnetic disk units without inducing adverse effects.
Related arts are disclosed in:
Patent Document 1 (Japanese Patent Application Laid-open No. 2004-30823);
Patent Document 2 (Japanese Patent Application Laid-open No. 2003-284362);
Patent Document 3 (Japanese Patent Application Laid-open No. 2003-61370);
Patent Document 4 (Japanese Patent Application Laid-open No. 2002-74871); and
Non-Patent Document 1 (JJAP, vol. 37, pp. 5306-5310, (1998)).